Modern radio communication systems such as LTE (LTE=long term evolution) being developed by 3GPP (3GPP=Third Generation Partnership) or WiMAX (WiMAX=Worldwide Interoperability for Microwave Access) being developed by IEEE (IEEE=Institute of Electrical and Electronics Engineers) and by WiMAX Forum use modulation techniques such as OFDM (OFDM=orthogonal frequency division multiplexing) with an enhanced spectral efficiency.
An alterable envelope of OFDM signals exhibit a large dynamic range. The large dynamic range provides high demands on digital-analog/analog digital converters regarding digital resolution and high demands according to a linear amplification of the OFDM signals by a power amplifier.
The dynamic range is defined by a peak-to-average-power ratio (PAPR=peak-to-average-power ratio), which is a ratio of a maximum instantaneous power to an average power.
The power an is designed to provide amplification for a highest arising value of an input power. Thereby, the power amplifier, being operated with a constant DC bias current (DC=direct current) of a power supply, works only with a highest efficiency according to the highest arising value of the input power or according to a so-called peak power. But most of the time, the input power is only a fraction of the peak power. This means, for long time periods an output power of the power amplifier is only a fraction of the power consumption of the power supply resulting in low power efficiency. Especially during time periods of low user traffic, such as time of night from midnight to 6 am, the power efficiency is quite low. Several solutions exist to improve the power efficiency of the power amplifier.
A PAPR reduction algorithm can be used to limit the peak power. In that way, the power amplifier is not designed according to the peak power but according to a pre-defined constant so-called clipping threshold, which is lower than the peak power. A so-called clipping removes signal peaks of the input signal, which are above the clipping threshold. Therewith, the power efficiency of the power amplifier can be improved. But an improvement can only be achieved for high load situations. During low load situations the efficiency is still quite low. A further disadvantage is a decreasing signal quality of the output signal of the power amplifier, because cutting the signal peaks increases an SNR (SNR=signal-to-noise-ratio) and increases the spectral bandwidth of the output signal.
An envelope tracking method is a further optimisation technique for the power amplifier by controlling the supply voltage of the power amplifier in real time depending on the current amplitude of the envelope of the input signal. Theoretically, a highest power efficiency of the power amplifier can be achieved by this technique. But complexity of transmitter design increases significantly because supplementary components such as coupler, detector, delay/synchronisation element, high-bandwidth voltage modulator or complex digital signal algorithm are required. In addition, a control of the envelope frocking method for a high bandwidth input signal such as 20 MHz bandwidth needs strong technical requirements on the supplementary components,
In DE 601 00 753 T2 a method for improving the efficiency and the dynamic range of a power amplifier is disclosed. A supply voltage is generated by a first power supply with low output voltage if an input signal is below a reference level and generated by a second power supply with high output voltage if the input signal is above the reference value.
The way of controlling the power supply of the power amplifier affects the power efficiency of the power amplifier.
Therefore, it is the object of the invention to improve the power efficiency of the power amplifier.